The invention disclosed and claimed herein is generally directed to a technique for magnetic resonance (MR) imaging, wherein a hyperpolarized noble gas or other agent is used to provide the population of magnetized spins or nuclei required for imaging.
The hyperpolarized magnetization technique has been found to be very useful in increasing the signal to noise ratio in certain MR imaging applications. Such applications include imaging of the lungs or other organ or tissue of a subject, wherein the structure to be imaged is proximate to cavities or air spaces. In accordance with one such application of the hyperpolarization technique, a noble gas such as optically pumped Helium-3 or Xenon-129 is hyperpolarized externally to the subject, and then introduced into the cavities or air spaces, such as by inhaling into the lungs. The magnetization of the hyperpolarized gaseous agent is substantially greater than the magnetization predicted by the Boltzmann distribution at the magnetic field strength and temperature of the imaging situation. Accordingly, the strength of acquired MR data signals, used to construct an MR image, tends to be substantially greater than for more well known MR techniques, wherein the magnetized spin population is provided by applying a very strong static or main magnetic field to the subject. Hyperpolarized magnetization is described, for example, in an article entitled "MR Imaging With Hyperpolarized .sup.3 He Gas", Middleton et al, published in Magnetic Resonance in Medicine, Vol. 33, No. 2, pp. 271-275 (1995).
In alternative implementations of the hyperpolarization technique, the requisite level of magnetization is provided by hyperpolarizing body structure, blood or other body fluid of the subject, as described hereinafter in greater detail.
In the hyperpolarization technique, as in other MR methods, a succession of RF excitation pulses are directed into a volume enclosing the portion of the subject to be imaged. The excitation pulses, together with corresponding magnetic gradient fields, act to generate the MR data signals for imaging. Each excitation pulse diminishes the hyperpolarized magnetization. However, after introduction into the subject, there is no mechanism available to restore the hyperpolarized magnetization lost by successive RF excitation, resulting in substantial reduction in the strength of later-acquired MR data signals. Instead, the longitudinal (T1) relaxation of the magnetization in the patient is to the Boltzmann (rather than hyperpolarized) value, which is much smaller, and can often be considered negligible.